Materials that exhibit adiabatic temperature change when subject to mechanical strain, magnetic fields, or electrical fields have been used to create heat pump cycles. A basic cycle is shown in FIG. 1. At state 1, a material is at steady temperature and is subject to a steady field applied directly to the material. An increase in the applied field strength increases material temperature at state 2. Heat is rejected to a hot ambient bringing the material temperature down near the hot ambient value in state 3. This is best accomplished through direct contact of the ambient air and the active material. Reduction of the field strength reduces material temperature at state 4. The cycle is then completed by absorbing heat from a cold ambient, again preferably through direct contact, causing the material temperature to rise back to the state 1 value. This cycle may approximate ideal Carnot, Brayton, or Ericsson cycles depending on the timing of field actuation in relation to heat rejection.
The adiabatic temperature lift available with known electrocaloric or magnetocaloric materials is typically lower than the lift required for most commercial heat pump applications such as environmental control. One well-known means of increasing temperature lift (at the expense of capacity) is temperature regeneration. Regeneration is used to develop a temperature gradient and thus multiply temperature lift in a regenerator that incorporates field-active material.
Regenerative heat exchangers are common in cycles that use fluid compression rather than field-active materials to provide heat pumping. For example, thermoacoustic coolers that apply a modified Stirling cycle are common practice. These units include one or more acoustic drivers, a resonant volume, a regenerator element and heat exchangers on either side of the element. The root of this technology is excitation of pressure and velocity fluctuations that compress and expand, as well as axially translate, the fluid within a regenerative heat exchanger. The fluid gives up heat to the regenerator matrix at one axial position when compressed and absorbs heat back at a different axial location when it is expanded. These heat exchanges create a temperature gradient shared by the regenerator matrix and the fluid within the regenerator. This gradient translates back and forth between hot and cold heat exchangers to pump heat in a manner similar to the field-activated regenerator case described above. The similarity is that the fluid within the regenerator is translated axially by some mechanical means. However, they differ in that in the field-active case the work for heat pumping comes entirely from the field imposed on the solid material of the regenerator and the fluid provides the heat capacity for regeneration, while in the thermoacoustic case the work for heat pumping comes from compression/expansion of the fluid within the regenerator and the solid material of the regenerator provides the heat capacity for regeneration. Also, in a thermoacoustic or other pressure-based cooling cycle, it is necessary to use a heat exchanger to separate the pressurized working fluid from the ambient air resulting in a significant loss in performance. Field-activated regenerators can be operated with the ambient air in direct contact with the active material.
The passive regenerator is known to benefit from several important performance characteristics. It must: 1) have adequate heat capacity in the solid media to store the energy to be regenerated; 2) allow passage of the working fluid without too much flow resistance; 3) enable heat transfer between the regenerator mass and working fluid; and 4) prevent heat conduction along the direction of the temperature gradient (and flow). Typical embodiments are cylindrical stacks created from layers of wire mesh or a duct filled with small metal spheres.